1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322
1323
1324
1325
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
1345
1346
1347
1348
1349
1350
1351
1352
1353
1354
1355
1356
1357
1358
1359
1360
1361
1362
1363
1364
1365
1366
1367
1368
1369
1370
1371
1372
1373
1374
1375
1376
1377
1378
1379
1380
1381
1382
1383
1384
1385
1386
1387
1388
1389
1390
1391
1392
1393
1394
1395
1396
1397
1398
1399
1400
1401
1402
1403
1404
1405
1406
1407
1408
1409
1410
1411
1412
1413
1414
1415
1416
1417
1418
1419
1420
1421
1422
1423
1424
1425
1426
1427
1428
1429
1430
1431
1432
1433
1434
1435
1436
1437
1438
1439
1440
1441
1442
1443
1444
1445
1446
1447
1448
1449
1450
1451
1452
1453
1454
1455
1456
1457
1458
1459
1460
1461
1462
1463
1464
1465
1466
1467
1468
1469
1470
1471
1472
1473
1474
1475
1476
1477
1478
1479
1480
1481
1482
1483
1484
1485
1486
1487
1488
1489
1490
1491
1492
1493
1494
1495
1496
1497
1498
1499
1500
1501
1502
1503
1504
1505
1506
1507
1508
1509
1510
1511
1512
1513
1514
1515
1516
1517
1518
1519
1520
1521
1522
1523
1524
1525
1526
1527
1528
1529
1530
1531
1532
1533
1534
1535
1536
1537
1538
1539
1540
1541
1542
1543
1544
1545
1546
1547
1548
1549
1550
1551
1552
1553
1554
1555
1556
1557
1558
1559
1560
1561
1562
1563
1564
1565
1566
1567
1568
1569
1570
1571
1572
1573
1574
1575
1576
1577
1578
1579
1580
1581
1582
1583
1584
1585
1586
1587
1588
1589
1590
1591
1592
1593
1594
1595
1596
1597
1598
1599
1600
1601
1602
1603
1604
1605
1606
1607
1608
1609
1610
1611
1612
1613
1614
1615
1616
1617
1618
1619
1620
1621
1622
1623
1624
1625
1626
1627
1628
1629
1630
1631
1632
1633
1634
1635
1636
1637
1638
1639
1640
1641
1642
1643
1644
1645
1646
1647
1648
1649
1650
1651
1652
1653
1654
1655
1656
1657
1658
1659
1660
1661
1662
1663
1664
1665
1666
1667
1668
1669
1670
1671
1672
1673
1674
1675
1676
1677
1678
1679
1680
1681
1682
1683
1684
1685
1686
1687
1688
1689
1690
1691
1692
1693
1694
1695
1696
1697
1698
1699
1700
1701
1702
1703
1704
1705
1706
1707
1708
1709
1710
1711
1712
1713
1714
1715
1716
1717
1718
1719
1720
1721
1722
1723
1724
1725
1726
1727
1728
1729
|
<!-- $PostgreSQL: pgsql/doc/src/sgml/backup.sgml,v 2.96 2007/01/31 20:56:16 momjian Exp $ -->
<chapter id="backup">
<title>Backup and Restore</title>
<indexterm zone="backup"><primary>backup</></>
<para>
As with everything that contains valuable data, <productname>PostgreSQL</>
databases should be backed up regularly. While the procedure is
essentially simple, it is important to have a basic understanding of
the underlying techniques and assumptions.
</para>
<para>
There are three fundamentally different approaches to backing up
<productname>PostgreSQL</> data:
<itemizedlist>
<listitem><para><acronym>SQL</> dump</para></listitem>
<listitem><para>File system level backup</para></listitem>
<listitem><para>Continuous archiving</para></listitem>
</itemizedlist>
Each has its own strengths and weaknesses.
</para>
<sect1 id="backup-dump">
<title><acronym>SQL</> Dump</title>
<para>
The idea behind this dump method is to generate a text file with SQL
commands that, when fed back to the server, will recreate the
database in the same state as it was at the time of the dump.
<productname>PostgreSQL</> provides the utility program
<xref linkend="app-pgdump"> for this purpose. The basic usage of this
command is:
<synopsis>
pg_dump <replaceable class="parameter">dbname</replaceable> > <replaceable class="parameter">outfile</replaceable>
</synopsis>
As you see, <application>pg_dump</> writes its results to the
standard output. We will see below how this can be useful.
</para>
<para>
<application>pg_dump</> is a regular <productname>PostgreSQL</>
client application (albeit a particularly clever one). This means
that you can do this backup procedure from any remote host that has
access to the database. But remember that <application>pg_dump</>
does not operate with special permissions. In particular, it must
have read access to all tables that you want to back up, so in
practice you almost always have to run it as a database superuser.
</para>
<para>
To specify which database server <application>pg_dump</> should
contact, use the command line options <option>-h
<replaceable>host</></> and <option>-p <replaceable>port</></>. The
default host is the local host or whatever your
<envar>PGHOST</envar> environment variable specifies. Similarly,
the default port is indicated by the <envar>PGPORT</envar>
environment variable or, failing that, by the compiled-in default.
(Conveniently, the server will normally have the same compiled-in
default.)
</para>
<para>
As any other <productname>PostgreSQL</> client application,
<application>pg_dump</> will by default connect with the database
user name that is equal to the current operating system user name. To override
this, either specify the <option>-U</option> option or set the
environment variable <envar>PGUSER</envar>. Remember that
<application>pg_dump</> connections are subject to the normal
client authentication mechanisms (which are described in <xref
linkend="client-authentication">).
</para>
<para>
Dumps created by <application>pg_dump</> are internally consistent,
that is, updates to the database while <application>pg_dump</> is
running will not be in the dump. <application>pg_dump</> does not
block other operations on the database while it is working.
(Exceptions are those operations that need to operate with an
exclusive lock, such as <command>VACUUM FULL</command>.)
</para>
<important>
<para>
If your database schema relies on OIDs (for instance as foreign
keys) you must instruct <application>pg_dump</> to dump the OIDs
as well. To do this, use the <option>-o</option> command line
option.
</para>
</important>
<sect2 id="backup-dump-restore">
<title>Restoring the dump</title>
<para>
The text files created by <application>pg_dump</> are intended to
be read in by the <application>psql</application> program. The
general command form to restore a dump is
<synopsis>
psql <replaceable class="parameter">dbname</replaceable> < <replaceable class="parameter">infile</replaceable>
</synopsis>
where <replaceable class="parameter">infile</replaceable> is what
you used as <replaceable class="parameter">outfile</replaceable>
for the <application>pg_dump</> command. The database <replaceable
class="parameter">dbname</replaceable> will not be created by this
command, so you must create it yourself from <literal>template0</>
before executing <application>psql</> (e.g., with
<literal>createdb -T template0 <replaceable
class="parameter">dbname</></literal>). <application>psql</>
supports similar options to <application>pg_dump</> for specifying
the database server to connect to and the user name to use. See
the <xref linkend="app-psql"> reference page for more information.
</para>
<para>
Before restoring a SQL dump, all the users who own objects or were
granted permissions on objects in the dumped database must already
exist. If they do not, then the restore will fail to recreate the
objects with the original ownership and/or permissions.
(Sometimes this is what you want, but usually it is not.)
</para>
<para>
By default, the <application>psql</> script will continue to
execute after an SQL error is encountered. You might wish to use the
following command at the top of the script to alter that
behaviour and have <application>psql</application> exit with an
exit status of 3 if an SQL error occurs:
<programlisting>
\set ON_ERROR_STOP
</programlisting>
Either way, you will only have a partially restored
dump. Alternatively, you can specify that the whole dump should be
restored as a single transaction, so the restore is either fully
completed or fully rolled back. This mode can be specified by
passing the <option>-1</> or <option>--single-transaction</>
command-line options to <application>psql</>. When using this
mode, be aware that even the smallest of errors can rollback a
restore that has already run for many hours. However, that might
still be preferable to manually cleaning up a complex database
after a partially restored dump.
</para>
<para>
The ability of <application>pg_dump</> and <application>psql</> to
write to or read from pipes makes it possible to dump a database
directly from one server to another; for example:
<programlisting>
pg_dump -h <replaceable>host1</> <replaceable>dbname</> | psql -h <replaceable>host2</> <replaceable>dbname</>
</programlisting>
</para>
<important>
<para>
The dumps produced by <application>pg_dump</> are relative to
<literal>template0</>. This means that any languages, procedures,
etc. added to <literal>template1</> will also be dumped by
<application>pg_dump</>. As a result, when restoring, if you are
using a customized <literal>template1</>, you must create the
empty database from <literal>template0</>, as in the example
above.
</para>
</important>
<para>
After restoring a backup, it is wise to run <xref
linkend="sql-analyze" endterm="sql-analyze-title"> on each
database so the query optimizer has useful statistics. An easy way
to do this is to run <command>vacuumdb -a -z</>; this is
equivalent to running <command>VACUUM ANALYZE</> on each database
manually. For more advice on how to load large amounts of data
into <productname>PostgreSQL</> efficiently, refer to <xref
linkend="populate">.
</para>
</sect2>
<sect2 id="backup-dump-all">
<title>Using <application>pg_dumpall</></title>
<para>
<application>pg_dump</> dumps only a single database at a time,
and it does not dump information about roles or tablespaces
(because those are cluster-wide rather than per-database).
To support convenient dumping of the entire contents of a database
cluster, the <xref linkend="app-pg-dumpall"> program is provided.
<application>pg_dumpall</> backs up each database in a given
cluster, and also preserves cluster-wide data such as role and
tablespace definitions. The basic usage of this command is:
<synopsis>
pg_dumpall > <replaceable>outfile</>
</synopsis>
The resulting dump can be restored with <application>psql</>:
<synopsis>
psql -f <replaceable class="parameter">infile</replaceable> postgres
</synopsis>
(Actually, you can specify any existing database name to start from,
but if you are reloading in an empty cluster then <literal>postgres</>
should generally be used.) It is always necessary to have
database superuser access when restoring a <application>pg_dumpall</>
dump, as that is required to restore the role and tablespace information.
If you use tablespaces, be careful that the tablespace paths in the
dump are appropriate for the new installation.
</para>
</sect2>
<sect2 id="backup-dump-large">
<title>Handling large databases</title>
<para>
Since <productname>PostgreSQL</productname> allows tables larger
than the maximum file size on your system, it can be problematic
to dump such a table to a file, since the resulting file will likely
be larger than the maximum size allowed by your system. Since
<application>pg_dump</> can write to the standard output, you can
use standard Unix tools to work around this possible problem.
</para>
<formalpara>
<title>Use compressed dumps.</title>
<para>
You can use your favorite compression program, for example
<application>gzip</application>.
<programlisting>
pg_dump <replaceable class="parameter">dbname</replaceable> | gzip > <replaceable class="parameter">filename</replaceable>.gz
</programlisting>
Reload with
<programlisting>
createdb <replaceable class="parameter">dbname</replaceable>
gunzip -c <replaceable class="parameter">filename</replaceable>.gz | psql <replaceable class="parameter">dbname</replaceable>
</programlisting>
or
<programlisting>
cat <replaceable class="parameter">filename</replaceable>.gz | gunzip | psql <replaceable class="parameter">dbname</replaceable>
</programlisting>
</para>
</formalpara>
<formalpara>
<title>Use <command>split</>.</title>
<para>
The <command>split</command> command
allows you to split the output into pieces that are
acceptable in size to the underlying file system. For example, to
make chunks of 1 megabyte:
<programlisting>
pg_dump <replaceable class="parameter">dbname</replaceable> | split -b 1m - <replaceable class="parameter">filename</replaceable>
</programlisting>
Reload with
<programlisting>
createdb <replaceable class="parameter">dbname</replaceable>
cat <replaceable class="parameter">filename</replaceable>* | psql <replaceable class="parameter">dbname</replaceable>
</programlisting>
</para>
</formalpara>
<formalpara>
<title>Use the custom dump format.</title>
<para>
If <productname>PostgreSQL</productname> was built on a system with the
<application>zlib</> compression library installed, the custom dump
format will compress data as it writes it to the output file. This will
produce dump file sizes similar to using <command>gzip</command>, but it
has the added advantage that tables can be restored selectively. The
following command dumps a database using the custom dump format:
<programlisting>
pg_dump -Fc <replaceable class="parameter">dbname</replaceable> > <replaceable class="parameter">filename</replaceable>
</programlisting>
A custom-format dump is not a script for <application>psql</>, but
instead must be restored with <application>pg_restore</>.
See the <xref linkend="app-pgdump"> and <xref
linkend="app-pgrestore"> reference pages for details.
</para>
</formalpara>
</sect2>
</sect1>
<sect1 id="backup-file">
<title>File System Level Backup</title>
<para>
An alternative backup strategy is to directly copy the files that
<productname>PostgreSQL</> uses to store the data in the database. In
<xref linkend="creating-cluster"> it is explained where these files
are located, but you have probably found them already if you are
interested in this method. You can use whatever method you prefer
for doing usual file system backups, for example
<programlisting>
tar -cf backup.tar /usr/local/pgsql/data
</programlisting>
</para>
<para>
There are two restrictions, however, which make this method
impractical, or at least inferior to the <application>pg_dump</>
method:
<orderedlist>
<listitem>
<para>
The database server <emphasis>must</> be shut down in order to
get a usable backup. Half-way measures such as disallowing all
connections will <emphasis>not</emphasis> work
(mainly because <command>tar</command> and similar tools do not take an
atomic snapshot of the state of the file system at a point in
time). Information about stopping the server can be found in
<xref linkend="server-shutdown">. Needless to say that you
also need to shut down the server before restoring the data.
</para>
</listitem>
<listitem>
<para>
If you have dug into the details of the file system layout of the
database, you might be tempted to try to back up or restore only certain
individual tables or databases from their respective files or
directories. This will <emphasis>not</> work because the
information contained in these files contains only half the
truth. The other half is in the commit log files
<filename>pg_clog/*</filename>, which contain the commit status of
all transactions. A table file is only usable with this
information. Of course it is also impossible to restore only a
table and the associated <filename>pg_clog</filename> data
because that would render all other tables in the database
cluster useless. So file system backups only work for complete
restoration of an entire database cluster.
</para>
</listitem>
</orderedlist>
</para>
<para>
An alternative file-system backup approach is to make a
<quote>consistent snapshot</quote> of the data directory, if the
file system supports that functionality (and you are willing to
trust that it is implemented correctly). The typical procedure is
to make a <quote>frozen snapshot</> of the volume containing the
database, then copy the whole data directory (not just parts, see
above) from the snapshot to a backup device, then release the frozen
snapshot. This will work even while the database server is running.
However, a backup created in this way saves
the database files in a state where the database server was not
properly shut down; therefore, when you start the database server
on the backed-up data, it will think the server had crashed
and replay the WAL log. This is not a problem, just be aware of
it (and be sure to include the WAL files in your backup).
</para>
<para>
If your database is spread across multiple file systems, there might not
be any way to obtain exactly-simultaneous frozen snapshots of all
the volumes. For example, if your data files and WAL log are on different
disks, or if tablespaces are on different file systems, it might
not be possible to use snapshot backup because the snapshots must be
simultaneous.
Read your file system documentation very carefully before trusting
to the consistent-snapshot technique in such situations. The safest
approach is to shut down the database server for long enough to
establish all the frozen snapshots.
</para>
<para>
Another option is to use <application>rsync</> to perform a file
system backup. This is done by first running <application>rsync</>
while the database server is running, then shutting down the database
server just long enough to do a second <application>rsync</>. The
second <application>rsync</> will be much quicker than the first,
because it has relatively little data to transfer, and the end result
will be consistent because the server was down. This method
allows a file system backup to be performed with minimal downtime.
</para>
<para>
Note that a file system backup will not necessarily be
smaller than an SQL dump. On the contrary, it will most likely be
larger. (<application>pg_dump</application> does not need to dump
the contents of indexes for example, just the commands to recreate
them.)
</para>
</sect1>
<sect1 id="continuous-archiving">
<title>Continuous Archiving and Point-In-Time Recovery (PITR)</title>
<indexterm zone="backup">
<primary>continuous archiving</primary>
</indexterm>
<indexterm zone="backup">
<primary>point-in-time recovery</primary>
</indexterm>
<indexterm zone="backup">
<primary>PITR</primary>
</indexterm>
<para>
At all times, <productname>PostgreSQL</> maintains a
<firstterm>write ahead log</> (WAL) in the <filename>pg_xlog/</>
subdirectory of the cluster's data directory. The log describes
every change made to the database's data files. This log exists
primarily for crash-safety purposes: if the system crashes, the
database can be restored to consistency by <quote>replaying</> the
log entries made since the last checkpoint. However, the existence
of the log makes it possible to use a third strategy for backing up
databases: we can combine a file-system-level backup with backup of
the WAL files. If recovery is needed, we restore the backup and
then replay from the backed-up WAL files to bring the backup up to
current time. This approach is more complex to administer than
either of the previous approaches, but it has some significant
benefits:
<itemizedlist>
<listitem>
<para>
We do not need a perfectly consistent backup as the starting point.
Any internal inconsistency in the backup will be corrected by log
replay (this is not significantly different from what happens during
crash recovery). So we don't need file system snapshot capability,
just <application>tar</> or a similar archiving tool.
</para>
</listitem>
<listitem>
<para>
Since we can string together an indefinitely long sequence of WAL files
for replay, continuous backup can be achieved simply by continuing to archive
the WAL files. This is particularly valuable for large databases, where
it might not be convenient to take a full backup frequently.
</para>
</listitem>
<listitem>
<para>
There is nothing that says we have to replay the WAL entries all the
way to the end. We could stop the replay at any point and have a
consistent snapshot of the database as it was at that time. Thus,
this technique supports <firstterm>point-in-time recovery</>: it is
possible to restore the database to its state at any time since your base
backup was taken.
</para>
</listitem>
<listitem>
<para>
If we continuously feed the series of WAL files to another
machine that has been loaded with the same base backup file, we
have a <firstterm>warm standby</> system: at any point we can bring up
the second machine and it will have a nearly-current copy of the
database.
</para>
</listitem>
</itemizedlist>
</para>
<para>
As with the plain file-system-backup technique, this method can only
support restoration of an entire database cluster, not a subset.
Also, it requires a lot of archival storage: the base backup might be bulky,
and a busy system will generate many megabytes of WAL traffic that
have to be archived. Still, it is the preferred backup technique in
many situations where high reliability is needed.
</para>
<para>
To recover successfully using continuous archiving (also called "online
backup" by many database vendors), you need a continuous
sequence of archived WAL files that extends back at least as far as the
start time of your backup. So to get started, you should setup and test
your procedure for archiving WAL files <emphasis>before</> you take your
first base backup. Accordingly, we first discuss the mechanics of
archiving WAL files.
</para>
<sect2 id="backup-archiving-wal">
<title>Setting up WAL archiving</title>
<para>
In an abstract sense, a running <productname>PostgreSQL</> system
produces an indefinitely long sequence of WAL records. The system
physically divides this sequence into WAL <firstterm>segment
files</>, which are normally 16MB apiece (although the size can be
altered when building <productname>PostgreSQL</>). The segment
files are given numeric names that reflect their position in the
abstract WAL sequence. When not using WAL archiving, the system
normally creates just a few segment files and then
<quote>recycles</> them by renaming no-longer-needed segment files
to higher segment numbers. It's assumed that a segment file whose
contents precede the checkpoint-before-last is no longer of
interest and can be recycled.
</para>
<para>
When archiving WAL data, we want to capture the contents of each segment
file once it is filled, and save that data somewhere before the segment
file is recycled for reuse. Depending on the application and the
available hardware, there could be many different ways of <quote>saving
the data somewhere</>: we could copy the segment files to an NFS-mounted
directory on another machine, write them onto a tape drive (ensuring that
you have a way of identifying the original name of each file), or batch
them together and burn them onto CDs, or something else entirely. To
provide the database administrator with as much flexibility as possible,
<productname>PostgreSQL</> tries not to make any assumptions about how
the archiving will be done. Instead, <productname>PostgreSQL</> lets
the administrator specify a shell command to be executed to copy a
completed segment file to wherever it needs to go. The command could be
as simple as a <literal>cp</>, or it could invoke a complex shell
script — it's all up to you.
</para>
<para>
The shell command to use is specified by the <xref
linkend="guc-archive-command"> configuration parameter, which in practice
will always be placed in the <filename>postgresql.conf</filename> file.
In this string,
any <literal>%p</> is replaced by the path name of the file to
archive, while any <literal>%f</> is replaced by the file name only.
(The path name is relative to the working directory of the server,
i.e., the cluster's data directory.)
Write <literal>%%</> if you need to embed an actual <literal>%</>
character in the command. The simplest useful command is something
like
<programlisting>
archive_command = 'cp -i %p /mnt/server/archivedir/%f </dev/null'
</programlisting>
which will copy archivable WAL segments to the directory
<filename>/mnt/server/archivedir</>. (This is an example, not a
recommendation, and might not work on all platforms.)
</para>
<para>
The archive command will be executed under the ownership of the same
user that the <productname>PostgreSQL</> server is running as. Since
the series of WAL files being archived contains effectively everything
in your database, you will want to be sure that the archived data is
protected from prying eyes; for example, archive into a directory that
does not have group or world read access.
</para>
<para>
It is important that the archive command return zero exit status if and
only if it succeeded. Upon getting a zero result,
<productname>PostgreSQL</> will assume that the WAL segment file has been
successfully archived, and will remove or recycle it.
However, a nonzero status tells
<productname>PostgreSQL</> that the file was not archived; it will try
again periodically until it succeeds.
</para>
<para>
The archive command should generally be designed to refuse to overwrite
any pre-existing archive file. This is an important safety feature to
preserve the integrity of your archive in case of administrator error
(such as sending the output of two different servers to the same archive
directory).
It is advisable to test your proposed archive command to ensure that it
indeed does not overwrite an existing file, <emphasis>and that it returns
nonzero status in this case</>. We have found that <literal>cp -i</> does
this correctly on some platforms but not others. If the chosen command
does not itself handle this case correctly, you should add a command
to test for pre-existence of the archive file. For example, something
like
<programlisting>
archive_command = 'test ! -f .../%f && cp %p .../%f'
</programlisting>
works correctly on most Unix variants.
</para>
<para>
While designing your archiving setup, consider what will happen if
the archive command fails repeatedly because some aspect requires
operator intervention or the archive runs out of space. For example, this
could occur if you write to tape without an autochanger; when the tape
fills, nothing further can be archived until the tape is swapped.
You should ensure that any error condition or request to a human operator
is reported appropriately so that the situation can be
resolved relatively quickly. The <filename>pg_xlog/</> directory will
continue to fill with WAL segment files until the situation is resolved.
</para>
<para>
The speed of the archiving command is not important, so long as it can keep up
with the average rate at which your server generates WAL data. Normal
operation continues even if the archiving process falls a little behind.
If archiving falls significantly behind, this will increase the amount of
data that would be lost in the event of a disaster. It will also mean that
the <filename>pg_xlog/</> directory will contain large numbers of
not-yet-archived segment files, which could eventually exceed available
disk space. You are advised to monitor the archiving process to ensure that
it is working as you intend.
</para>
<para>
In writing your archive command, you should assume that the file names to
be archived can be up to 64 characters long and can contain any
combination of ASCII letters, digits, and dots. It is not necessary to
remember the original relative path (<literal>%p</>) but it is necessary to
remember the file name (<literal>%f</>).
</para>
<para>
Note that although WAL archiving will allow you to restore any
modifications made to the data in your <productname>PostgreSQL</> database,
it will not restore changes made to configuration files (that is,
<filename>postgresql.conf</>, <filename>pg_hba.conf</> and
<filename>pg_ident.conf</>), since those are edited manually rather
than through SQL operations.
You might wish to keep the configuration files in a location that will
be backed up by your regular file system backup procedures. See
<xref linkend="runtime-config-file-locations"> for how to relocate the
configuration files.
</para>
<para>
The archive command is only invoked on completed WAL segments. Hence,
if your server generates only little WAL traffic (or has slack periods
where it does so), there could be a long delay between the completion
of a transaction and its safe recording in archive storage. To put
a limit on how old unarchived data can be, you can set
<xref linkend="guc-archive-timeout"> to force the server to switch
to a new WAL segment file at least that often. Note that archived
files that are ended early due to a forced switch are still the same
length as completely full files. It is therefore unwise to set a very
short <varname>archive_timeout</> — it will bloat your archive
storage. <varname>archive_timeout</> settings of a minute or so are
usually reasonable.
</para>
<para>
Also, you can force a segment switch manually with
<function>pg_switch_xlog</>, if you want to ensure that a
just-finished transaction is archived immediately. Other utility
functions related to WAL management are listed in <xref
linkend="functions-admin-backup-table">.
</para>
</sect2>
<sect2 id="backup-base-backup">
<title>Making a Base Backup</title>
<para>
The procedure for making a base backup is relatively simple:
<orderedlist>
<listitem>
<para>
Ensure that WAL archiving is enabled and working.
</para>
</listitem>
<listitem>
<para>
Connect to the database as a superuser, and issue the command
<programlisting>
SELECT pg_start_backup('label');
</programlisting>
where <literal>label</> is any string you want to use to uniquely
identify this backup operation. (One good practice is to use the
full path where you intend to put the backup dump file.)
<function>pg_start_backup</> creates a <firstterm>backup label</> file,
called <filename>backup_label</>, in the cluster directory with
information about your backup.
</para>
<para>
It does not matter which database within the cluster you connect to to
issue this command. You can ignore the result returned by the function;
but if it reports an error, deal with that before proceeding.
</para>
</listitem>
<listitem>
<para>
Perform the backup, using any convenient file-system-backup tool
such as <application>tar</> or <application>cpio</>. It is neither
necessary nor desirable to stop normal operation of the database
while you do this.
</para>
</listitem>
<listitem>
<para>
Again connect to the database as a superuser, and issue the command
<programlisting>
SELECT pg_stop_backup();
</programlisting>
This terminates the backup mode and performs an automatic switch to
the next WAL segment. The reason for the switch is to arrange that
the last WAL segment file written during the backup interval is
immediately ready to archive.
</para>
</listitem>
<listitem>
<para>
Once the WAL segment files used during the backup are archived, you are
done. The file identified by <function>pg_stop_backup</>'s result is
the last segment that needs to be archived to complete the backup.
Archival of these files will happen automatically, since you have
already configured <varname>archive_command</>. In many cases, this
happens fairly quickly, but you are advised to monitor your archival
system to ensure this has taken place so that you can be certain you
have a complete backup.
</para>
</listitem>
</orderedlist>
</para>
<para>
Some backup tools that you might wish to use emit warnings or errors
if the files they are trying to copy change while the copy proceeds.
This situation is normal, and not an error, when taking a base backup of
an active database; so you need to ensure that you can distinguish
complaints of this sort from real errors. For example, some versions
of <application>rsync</> return a separate exit code for <quote>vanished
source files</>, and you can write a driver script to accept this exit
code as a non-error case. Also,
some versions of GNU <application>tar</> consider it an error if a file
is changed while <application>tar</> is copying it. There does not seem
to be any very convenient way to distinguish this error from other types
of errors, other than manual inspection of <application>tar</>'s messages.
GNU <application>tar</> is therefore not the best tool for making base
backups.
</para>
<para>
It is not necessary to be very concerned about the amount of time elapsed
between <function>pg_start_backup</> and the start of the actual backup,
nor between the end of the backup and <function>pg_stop_backup</>; a
few minutes' delay won't hurt anything. (However, if you normally run the
server with <varname>full_page_writes</> disabled, you might notice a drop
in performance between <function>pg_start_backup</> and
<function>pg_stop_backup</>, since <varname>full_page_writes</> is
effectively forced on during backup mode.) You must ensure that these
steps are carried out in sequence without any possible
overlap, or you will invalidate the backup.
</para>
<para>
Be certain that your backup dump includes all of the files underneath
the database cluster directory (e.g., <filename>/usr/local/pgsql/data</>).
If you are using tablespaces that do not reside underneath this directory,
be careful to include them as well (and be sure that your backup dump
archives symbolic links as links, otherwise the restore will mess up
your tablespaces).
</para>
<para>
You can, however, omit from the backup dump the files within the
<filename>pg_xlog/</> subdirectory of the cluster directory. This
slight complication is worthwhile because it reduces the risk
of mistakes when restoring. This is easy to arrange if
<filename>pg_xlog/</> is a symbolic link pointing to someplace outside
the cluster directory, which is a common setup anyway for performance
reasons.
</para>
<para>
To make use of the backup, you will need to keep around all the WAL
segment files generated during and after the file system backup.
To aid you in doing this, the <function>pg_stop_backup</> function
creates a <firstterm>backup history file</> that is immediately
stored into the WAL archive area. This file is named after the first
WAL segment file that you need to have to make use of the backup.
For example, if the starting WAL file is
<literal>0000000100001234000055CD</> the backup history file will be
named something like
<literal>0000000100001234000055CD.007C9330.backup</>. (The second
number in the file name stands for an exact position within the WAL
file, and can ordinarily be ignored.) Once you have safely archived
the file system backup and the WAL segment files used during the
backup (as specified in the backup history file), all archived WAL
segments with names numerically less are no longer needed to recover
the file system backup and can be deleted. However, you should
consider keeping several backup sets to be absolutely certain that
you can recover your data.
</para>
<para>
The backup history file is just a small text file. It contains the
label string you gave to <function>pg_start_backup</>, as well as
the starting and ending times and WAL segments of the backup.
If you used the label to identify where the associated dump file is kept,
then the archived history file is enough to tell you which dump file to
restore, should you need to do so.
</para>
<para>
Since you have to keep around all the archived WAL files back to your
last base backup, the interval between base backups should usually be
chosen based on how much storage you want to expend on archived WAL
files. You should also consider how long you are prepared to spend
recovering, if recovery should be necessary — the system will have to
replay all those WAL segments, and that could take awhile if it has
been a long time since the last base backup.
</para>
<para>
It's also worth noting that the <function>pg_start_backup</> function
makes a file named <filename>backup_label</> in the database cluster
directory, which is then removed again by <function>pg_stop_backup</>.
This file will of course be archived as a part of your backup dump file.
The backup label file includes the label string you gave to
<function>pg_start_backup</>, as well as the time at which
<function>pg_start_backup</> was run, and the name of the starting WAL
file. In case of confusion it will
therefore be possible to look inside a backup dump file and determine
exactly which backup session the dump file came from.
</para>
<para>
It is also possible to make a backup dump while the server is
stopped. In this case, you obviously cannot use
<function>pg_start_backup</> or <function>pg_stop_backup</>, and
you will therefore be left to your own devices to keep track of which
backup dump is which and how far back the associated WAL files go.
It is generally better to follow the continuous archiving procedure above.
</para>
</sect2>
<sect2 id="backup-pitr-recovery">
<title>Recovering using a Continuous Archive Backup</title>
<para>
Okay, the worst has happened and you need to recover from your backup.
Here is the procedure:
<orderedlist>
<listitem>
<para>
Stop the server, if it's running.
</para>
</listitem>
<listitem>
<para>
If you have the space to do so,
copy the whole cluster data directory and any tablespaces to a temporary
location in case you need them later. Note that this precaution will
require that you have enough free space on your system to hold two
copies of your existing database. If you do not have enough space,
you need at the least to copy the contents of the <filename>pg_xlog</>
subdirectory of the cluster data directory, as it might contain logs which
were not archived before the system went down.
</para>
</listitem>
<listitem>
<para>
Clean out all existing files and subdirectories under the cluster data
directory and under the root directories of any tablespaces you are using.
</para>
</listitem>
<listitem>
<para>
Restore the database files from your backup dump. Be careful that they
are restored with the right ownership (the database system user, not
root!) and with the right permissions. If you are using tablespaces,
you should verify that the symbolic links in <filename>pg_tblspc/</>
were correctly restored.
</para>
</listitem>
<listitem>
<para>
Remove any files present in <filename>pg_xlog/</>; these came from the
backup dump and are therefore probably obsolete rather than current.
If you didn't archive <filename>pg_xlog/</> at all, then recreate it,
and be sure to recreate the subdirectory
<filename>pg_xlog/archive_status/</> as well.
</para>
</listitem>
<listitem>
<para>
If you had unarchived WAL segment files that you saved in step 2,
copy them into <filename>pg_xlog/</>. (It is best to copy them,
not move them, so that you still have the unmodified files if a
problem occurs and you have to start over.)
</para>
</listitem>
<listitem>
<para>
Create a recovery command file <filename>recovery.conf</> in the cluster
data directory (see <xref linkend="recovery-config-settings">). You might
also want to temporarily modify <filename>pg_hba.conf</> to prevent
ordinary users from connecting until you are sure the recovery has worked.
</para>
</listitem>
<listitem>
<para>
Start the server. The server will go into recovery mode and
proceed to read through the archived WAL files it needs. Should the
recovery be terminated because of an external error, the server can
simply be restarted and it will continue recovery. Upon completion
of the recovery process, the server will rename
<filename>recovery.conf</> to <filename>recovery.done</> (to prevent
accidentally re-entering recovery mode in case of a crash later) and then
commence normal database operations.
</para>
</listitem>
<listitem>
<para>
Inspect the contents of the database to ensure you have recovered to
where you want to be. If not, return to step 1. If all is well,
let in your users by restoring <filename>pg_hba.conf</> to normal.
</para>
</listitem>
</orderedlist>
</para>
<para>
The key part of all this is to setup a recovery command file that
describes how you want to recover and how far the recovery should
run. You can use <filename>recovery.conf.sample</> (normally
installed in the installation <filename>share/</> directory) as a
prototype. The one thing that you absolutely must specify in
<filename>recovery.conf</> is the <varname>restore_command</>,
which tells <productname>PostgreSQL</> how to get back archived
WAL file segments. Like the <varname>archive_command</>, this is
a shell command string. It can contain <literal>%f</>, which is
replaced by the name of the desired log file, and <literal>%p</>,
which is replaced by the path name to copy the log file to.
(The path name is relative to the working directory of the server,
i.e., the cluster's data directory.)
Write <literal>%%</> if you need to embed an actual <literal>%</>
character in the command. The simplest useful command is
something like
<programlisting>
restore_command = 'cp /mnt/server/archivedir/%f %p'
</programlisting>
which will copy previously archived WAL segments from the directory
<filename>/mnt/server/archivedir</>. You could of course use something
much more complicated, perhaps even a shell script that requests the
operator to mount an appropriate tape.
</para>
<para>
It is important that the command return nonzero exit status on failure.
The command <emphasis>will</> be asked for log files that are not present
in the archive; it must return nonzero when so asked. This is not an
error condition. Be aware also that the base name of the <literal>%p</>
path will be different from <literal>%f</>; do not expect them to be
interchangeable.
</para>
<para>
WAL segments that cannot be found in the archive will be sought in
<filename>pg_xlog/</>; this allows use of recent un-archived segments.
However segments that are available from the archive will be used in
preference to files in <filename>pg_xlog/</>. The system will not
overwrite the existing contents of <filename>pg_xlog/</> when retrieving
archived files.
</para>
<para>
Normally, recovery will proceed through all available WAL segments,
thereby restoring the database to the current point in time (or as
close as we can get given the available WAL segments). But if you want
to recover to some previous point in time (say, right before the junior
DBA dropped your main transaction table), just specify the required
stopping point in <filename>recovery.conf</>. You can specify the stop
point, known as the <quote>recovery target</>, either by date/time or
by completion of a specific transaction ID. As of this writing only
the date/time option is very usable, since there are no tools to help
you identify with any accuracy which transaction ID to use.
</para>
<note>
<para>
The stop point must be after the ending time of the base backup (the
time of <function>pg_stop_backup</>). You cannot use a base backup
to recover to a time when that backup was still going on. (To
recover to such a time, you must go back to your previous base backup
and roll forward from there.)
</para>
</note>
<para>
If recovery finds a corruption in the WAL data then recovery will
complete at that point and the server will not start. In such a case the
recovery process could be re-run from the beginning, specifying a
<quote>recovery target</> before the point of corruption so that recovery
can complete normally.
If recovery fails for an external reason, such as a system crash or
if the WAL archive has become inaccessible, then the recovery can simply
be restarted and it will restart almost from where it failed.
Recovery restart works much like checkpointing in normal operation:
the server periodically forces all its state to disk, and then updates
the <filename>pg_control</> file to indicate that the already-processed
WAL data need not be scanned again.
</para>
<sect3 id="recovery-config-settings" xreflabel="Recovery Settings">
<title>Recovery Settings</title>
<para>
These settings can only be made in the <filename>recovery.conf</>
file, and apply only for the duration of the recovery. They must be
reset for any subsequent recovery you wish to perform. They cannot be
changed once recovery has begun.
</para>
<variablelist>
<varlistentry id="restore-command" xreflabel="restore_command">
<term><varname>restore_command</varname> (<type>string</type>)</term>
<listitem>
<para>
The shell command to execute to retrieve an archived segment of
the WAL file series. This parameter is required.
Any <literal>%f</> in the string is
replaced by the name of the file to retrieve from the archive,
and any <literal>%p</> is replaced by the path name to copy
it to on the server.
(The path name is relative to the working directory of the server,
i.e., the cluster's data directory.)
Write <literal>%%</> to embed an actual <literal>%</> character
in the command.
</para>
<para>
It is important for the command to return a zero exit status if and
only if it succeeds. The command <emphasis>will</> be asked for file
names that are not present in the archive; it must return nonzero
when so asked. Examples:
<programlisting>
restore_command = 'cp /mnt/server/archivedir/%f "%p"'
restore_command = 'copy /mnt/server/archivedir/%f "%p"' # Windows
</programlisting>
</para>
</listitem>
</varlistentry>
<varlistentry id="recovery-target-time" xreflabel="recovery_target_time">
<term><varname>recovery_target_time</varname>
(<type>timestamp</type>)
</term>
<listitem>
<para>
This parameter specifies the time stamp up to which recovery
will proceed.
At most one of <varname>recovery_target_time</> and
<xref linkend="recovery-target-xid"> can be specified.
The default is to recover to the end of the WAL log.
The precise stopping point is also influenced by
<xref linkend="recovery-target-inclusive">.
</para>
</listitem>
</varlistentry>
<varlistentry id="recovery-target-xid" xreflabel="recovery_target_xid">
<term><varname>recovery_target_xid</varname> (<type>string</type>)</term>
<listitem>
<para>
This parameter specifies the transaction ID up to which recovery
will proceed. Keep in mind
that while transaction IDs are assigned sequentially at transaction
start, transactions can complete in a different numeric order.
The transactions that will be recovered are those that committed
before (and optionally including) the specified one.
At most one of <varname>recovery_target_xid</> and
<xref linkend="recovery-target-time"> can be specified.
The default is to recover to the end of the WAL log.
The precise stopping point is also influenced by
<xref linkend="recovery-target-inclusive">.
</para>
</listitem>
</varlistentry>
<varlistentry id="recovery-target-inclusive"
xreflabel="recovery_target_inclusive">
<term><varname>recovery_target_inclusive</varname>
(<type>boolean</type>)
</term>
<listitem>
<para>
Specifies whether we stop just after the specified recovery target
(<literal>true</literal>), or just before the recovery target
(<literal>false</literal>).
Applies to both <xref linkend="recovery-target-time">
and <xref linkend="recovery-target-xid">, whichever one is
specified for this recovery. This indicates whether transactions
having exactly the target commit time or ID, respectively, will
be included in the recovery. Default is <literal>true</>.
</para>
</listitem>
</varlistentry>
<varlistentry id="recovery-target-timeline"
xreflabel="recovery_target_timeline">
<term><varname>recovery_target_timeline</varname>
(<type>string</type>)
</term>
<listitem>
<para>
Specifies recovering into a particular timeline. The default is
to recover along the same timeline that was current when the
base backup was taken. You would only need to set this parameter
in complex re-recovery situations, where you need to return to
a state that itself was reached after a point-in-time recovery.
See <xref linkend="backup-timelines"> for discussion.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect3>
</sect2>
<sect2 id="backup-timelines">
<title>Timelines</title>
<indexterm zone="backup">
<primary>timelines</primary>
</indexterm>
<para>
The ability to restore the database to a previous point in time creates
some complexities that are akin to science-fiction stories about time
travel and parallel universes. In the original history of the database,
perhaps you dropped a critical table at 5:15PM on Tuesday evening.
Unfazed, you get out your backup, restore to the point-in-time 5:14PM
Tuesday evening, and are up and running. In <emphasis>this</> history of
the database universe, you never dropped the table at all. But suppose
you later realize this wasn't such a great idea after all, and would like
to return to some later point in the original history. You won't be able
to if, while your database was up-and-running, it overwrote some of the
sequence of WAL segment files that led up to the time you now wish you
could get back to. So you really want to distinguish the series of
WAL records generated after you've done a point-in-time recovery from
those that were generated in the original database history.
</para>
<para>
To deal with these problems, <productname>PostgreSQL</> has a notion
of <firstterm>timelines</>. Each time you recover to a point-in-time
earlier than the end of the WAL sequence, a new timeline is created
to identify the series of WAL records generated after that recovery.
(If recovery proceeds all the way to the end of WAL, however, we do not
start a new timeline: we just extend the existing one.) The timeline
ID number is part of WAL segment file names, and so a new timeline does
not overwrite the WAL data generated by previous timelines. It is
in fact possible to archive many different timelines. While that might
seem like a useless feature, it's often a lifesaver. Consider the
situation where you aren't quite sure what point-in-time to recover to,
and so have to do several point-in-time recoveries by trial and error
until you find the best place to branch off from the old history. Without
timelines this process would soon generate an unmanageable mess. With
timelines, you can recover to <emphasis>any</> prior state, including
states in timeline branches that you later abandoned.
</para>
<para>
Each time a new timeline is created, <productname>PostgreSQL</> creates
a <quote>timeline history</> file that shows which timeline it branched
off from and when. These history files are necessary to allow the system
to pick the right WAL segment files when recovering from an archive that
contains multiple timelines. Therefore, they are archived into the WAL
archive area just like WAL segment files. The history files are just
small text files, so it's cheap and appropriate to keep them around
indefinitely (unlike the segment files which are large). You can, if
you like, add comments to a history file to make your own notes about
how and why this particular timeline came to be. Such comments will be
especially valuable when you have a thicket of different timelines as
a result of experimentation.
</para>
<para>
The default behavior of recovery is to recover along the same timeline
that was current when the base backup was taken. If you want to recover
into some child timeline (that is, you want to return to some state that
was itself generated after a recovery attempt), you need to specify the
target timeline ID in <filename>recovery.conf</>. You cannot recover into
timelines that branched off earlier than the base backup.
</para>
</sect2>
<sect2 id="continuous-archiving-caveats">
<title>Caveats</title>
<para>
At this writing, there are several limitations of the continuous archiving
technique. These will probably be fixed in future releases:
<itemizedlist>
<listitem>
<para>
Operations on hash indexes are not presently WAL-logged, so
replay will not update these indexes. The recommended workaround
is to manually <xref linkend="sql-reindex" endterm="sql-reindex-title">
each such index after completing a recovery operation.
</para>
</listitem>
<listitem>
<para>
If a <xref linkend="sql-createdatabase" endterm="sql-createdatabase-title">
command is executed while a base backup is being taken, and then
the template database that the <command>CREATE DATABASE</> copied
is modified while the base backup is still in progress, it is
possible that recovery will cause those modifications to be
propagated into the created database as well. This is of course
undesirable. To avoid this risk, it is best not to modify any
template databases while taking a base backup.
</para>
</listitem>
<listitem>
<para>
<xref linkend="sql-createtablespace" endterm="sql-createtablespace-title">
commands are WAL-logged with the literal absolute path, and will
therefore be replayed as tablespace creations with the same
absolute path. This might be undesirable if the log is being
replayed on a different machine. It can be dangerous even if the
log is being replayed on the same machine, but into a new data
directory: the replay will still overwrite the contents of the
original tablespace. To avoid potential gotchas of this sort,
the best practice is to take a new base backup after creating or
dropping tablespaces.
</para>
</listitem>
</itemizedlist>
</para>
<para>
It should also be noted that the default <acronym>WAL</acronym>
format is fairly bulky since it includes many disk page snapshots.
These page snapshots are designed to support crash recovery, since
we might need to fix partially-written disk pages. Depending on
your system hardware and software, the risk of partial writes might
be small enough to ignore, in which case you can significantly
reduce the total volume of archived logs by turning off page
snapshots using the <xref linkend="guc-full-page-writes">
parameter. (Read the notes and warnings in <xref linkend="wal">
before you do so.) Turning off page snapshots does not prevent
use of the logs for PITR operations. An area for future
development is to compress archived WAL data by removing
unnecessary page copies even when <varname>full_page_writes</> is
on. In the meantime, administrators might wish to reduce the number
of page snapshots included in WAL by increasing the checkpoint
interval parameters as much as feasible.
</para>
</sect2>
</sect1>
<sect1 id="warm-standby">
<title>Warm Standby Servers for High Availability</title>
<indexterm zone="backup">
<primary>warm standby</primary>
</indexterm>
<indexterm zone="backup">
<primary>PITR standby</primary>
</indexterm>
<indexterm zone="backup">
<primary>standby server</primary>
</indexterm>
<indexterm zone="backup">
<primary>log shipping</primary>
</indexterm>
<indexterm zone="backup">
<primary>witness server</primary>
</indexterm>
<indexterm zone="backup">
<primary>STONITH</primary>
</indexterm>
<indexterm zone="backup">
<primary>high availability</primary>
</indexterm>
<para>
Continuous archiving can be used to create a <firstterm>high
availability</> (HA) cluster configuration with one or more
<firstterm>standby servers</> ready to take
over operations if the primary server fails. This
capability is widely referred to as <firstterm>warm standby</>
or <firstterm>log shipping</>.
</para>
<para>
The primary and standby server work together to provide this capability,
though the servers are only loosely coupled. The primary server operates
in continuous archiving mode, while each standby server operates in
continuous recovery mode, reading the WAL files from the primary. No
changes to the database tables are required to enable this capability,
so it offers low administration overhead in comparison with some other
replication approaches. This configuration also has relatively low
performance impact on the primary server.
</para>
<para>
Directly moving WAL or "log" records from one database server to another
is typically described as log shipping. <productname>PostgreSQL</>
implements file-based log shipping, which means that WAL records are
transferred one file (WAL segment) at a time. WAL
files can be shipped easily and cheaply over any distance, whether it be
to an adjacent system, another system on the same site or another system
on the far side of the globe. The bandwidth required for this technique
varies according to the transaction rate of the primary server.
Record-based log shipping is also possible with custom-developed
procedures, as discussed in <xref linkend="warm-standby-record">.
</para>
<para>
It should be noted that the log shipping is asynchronous, i.e. the
WAL records are shipped after transaction commit. As a result there
is a window for data loss should the primary server
suffer a catastrophic failure: transactions not yet shipped will be lost.
The length of the window of data loss
can be limited by use of the <varname>archive_timeout</varname> parameter,
which can be set as low as a few seconds if required. However such low
settings will substantially increase the bandwidth requirements for file
shipping. If you need a window of less than a minute or so, it's probably
better to look into record-based log shipping.
</para>
<para>
The standby server is not available for access, since it is continually
performing recovery processing. Recovery performance is sufficiently
good that the standby will typically be only moments away from full
availability once it has been activated. As a result, we refer to this
capability as a warm standby configuration that offers high
availability. Restoring a server from an archived base backup and
rollforward will take considerably longer, so that technique only
really offers a solution for disaster recovery, not HA.
</para>
<sect2 id="warm-standby-planning">
<title>Planning</title>
<para>
It is usually wise to create the primary and standby servers
so that they are as similar as possible, at least from the
perspective of the database server. In particular, the path names
associated with tablespaces will be passed across as-is, so both
primary and standby servers must have the same mount paths for
tablespaces if that feature is used. Keep in mind that if
<xref linkend="sql-createtablespace" endterm="sql-createtablespace-title">
is executed on the primary, any new mount point needed for it must
be created on both the primary and all standby servers before the command
is executed. Hardware need not be exactly the same, but experience shows
that maintaining two identical systems is easier than maintaining two
dissimilar ones over the lifetime of the application and system.
In any case the hardware architecture must be the same — shipping
from, say, a 32-bit to a 64-bit system will not work.
</para>
<para>
In general, log shipping between servers running different major release
levels will not be possible. It is the policy of the PostgreSQL Global
Development Group not to make changes to disk formats during minor release
upgrades, so it is likely that running different minor release levels
on primary and standby servers will work successfully. However, no
formal support for that is offered and you are advised to keep primary
and standby servers at the same release level as much as possible.
When updating to a new minor release, the safest policy is to update
the standby servers first — a new minor release is more likely
to be able to read WAL files from a previous minor release than vice
versa.
</para>
<para>
There is no special mode required to enable a standby server. The
operations that occur on both primary and standby servers are entirely
normal continuous archiving and recovery tasks. The only point of
contact between the two database servers is the archive of WAL files
that both share: primary writing to the archive, standby reading from
the archive. Care must be taken to ensure that WAL archives for separate
primary servers do not become mixed together or confused.
</para>
<para>
The magic that makes the two loosely coupled servers work together
is simply a <varname>restore_command</> used on the standby that waits for
the next WAL file to become available from the primary. The
<varname>restore_command</> is specified in the <filename>recovery.conf</>
file on the standby
server. Normal recovery processing would request a file from the
WAL archive, reporting failure if the file was unavailable. For
standby processing it is normal for the next file to be
unavailable, so we must be patient and wait for it to appear. A
waiting <varname>restore_command</> can be written as a custom
script that loops after polling for the existence of the next WAL
file. There must also be some way to trigger failover, which
should interrupt the <varname>restore_command</>, break the loop
and return a file-not-found error to the standby server. This
ends recovery and the standby will then come up as a normal
server.
</para>
<para>
Pseudocode for a suitable <varname>restore_command</> is:
<programlisting>
triggered = false;
while (!NextWALFileReady() && !triggered)
{
sleep(100000L); /* wait for ~0.1 sec */
if (CheckForExternalTrigger())
triggered = true;
}
if (!triggered)
CopyWALFileForRecovery();
</programlisting>
</para>
<para>
<productname>PostgreSQL</productname> does not provide the system
software required to identify a failure on the primary and notify
the standby system and then the standby database server. Many such
tools exist and are well integrated with other aspects required for
successful failover, such as IP address migration.
</para>
<para>
The means for triggering failover is an important part of planning and
design. The <varname>restore_command</> is executed in full once
for each WAL file. The process running the <varname>restore_command</>
is therefore created and dies for each file, so there is no daemon
or server process and so we cannot use signals and a signal
handler. A more permanent notification is required to trigger the
failover. It is possible to use a simple timeout facility,
especially if used in conjunction with a known
<varname>archive_timeout</> setting on the primary. This is
somewhat error prone since a network problem or busy primary server might
be sufficient to initiate failover. A notification mechanism such
as the explicit creation of a trigger file is less error prone, if
this can be arranged.
</para>
</sect2>
<sect2 id="warm-standby-config">
<title>Implementation</title>
<para>
The short procedure for configuring a standby server is as follows. For
full details of each step, refer to previous sections as noted.
<orderedlist>
<listitem>
<para>
Set up primary and standby systems as near identically as
possible, including two identical copies of
<productname>PostgreSQL</> at the same release level.
</para>
</listitem>
<listitem>
<para>
Set up continuous archiving from the primary to a WAL archive located
in a directory on the standby server. Ensure that <xref
linkend="guc-archive-command"> and <xref linkend="guc-archive-timeout">
are set appropriately on the primary
(see <xref linkend="backup-archiving-wal">).
</para>
</listitem>
<listitem>
<para>
Make a base backup of the primary server (see <xref
linkend="backup-base-backup">), and load this data onto the standby.
</para>
</listitem>
<listitem>
<para>
Begin recovery on the standby server from the local WAL
archive, using a <filename>recovery.conf</> that specifies a
<varname>restore_command</> that waits as described
previously (see <xref linkend="backup-pitr-recovery">).
</para>
</listitem>
</orderedlist>
</para>
<para>
Recovery treats the WAL archive as read-only, so once a WAL file has
been copied to the standby system it can be copied to tape at the same
time as it is being read by the standby database server.
Thus, running a standby server for high availability can be performed at
the same time as files are stored for longer term disaster recovery
purposes.
</para>
<para>
For testing purposes, it is possible to run both primary and standby
servers on the same system. This does not provide any worthwhile
improvement in server robustness, nor would it be described as HA.
</para>
</sect2>
<sect2 id="warm-standby-failover">
<title>Failover</title>
<para>
If the primary server fails then the standby server should begin
failover procedures.
</para>
<para>
If the standby server fails then no failover need take place. If the
standby server can be restarted, even some time later, then the recovery
process can also be immediately restarted, taking advantage of
restartable recovery. If the standby server cannot be restarted, then a
full new standby server should be created.
</para>
<para>
If the primary server fails and then immediately restarts, you must have
a mechanism for informing it that it is no longer the primary. This is
sometimes known as STONITH (Shoot the Other Node In The Head), which is
necessary to avoid situations where both systems think they are the
primary, which can lead to confusion and ultimately data loss.
</para>
<para>
Many failover systems use just two systems, the primary and the standby,
connected by some kind of heartbeat mechanism to continually verify the
connectivity between the two and the viability of the primary. It is
also possible to use a third system (called a witness server) to avoid
some problems of inappropriate failover, but the additional complexity
might not be worthwhile unless it is set-up with sufficient care and
rigorous testing.
</para>
<para>
Once failover to the standby occurs, we have only a
single server in operation. This is known as a degenerate state.
The former standby is now the primary, but the former primary is down
and might stay down. To return to normal operation we must
fully recreate a standby server,
either on the former primary system when it comes up, or on a third,
possibly new, system. Once complete the primary and standby can be
considered to have switched roles. Some people choose to use a third
server to provide backup to the new primary until the new standby
server is recreated,
though clearly this complicates the system configuration and
operational processes.
</para>
<para>
So, switching from primary to standby server can be fast but requires
some time to re-prepare the failover cluster. Regular switching from
primary to standby is encouraged, since it allows regular downtime on
each system for maintenance. This also acts as a test of the
failover mechanism to ensure that it will really work when you need it.
Written administration procedures are advised.
</para>
</sect2>
<sect2 id="warm-standby-record">
<title>Record-based Log Shipping</title>
<para>
<productname>PostgreSQL</productname> directly supports file-based
log shipping as described above. It is also possible to implement
record-based log shipping, though this requires custom development.
</para>
<para>
An external program can call the <function>pg_xlogfile_name_offset()</>
function (see <xref linkend="functions-admin">)
to find out the file name and the exact byte offset within it of
the current end of WAL. It can then access the WAL file directly
and copy the data from the last known end of WAL through the current end
over to the standby server(s). With this approach, the window for data
loss is the polling cycle time of the copying program, which can be very
small, but there is no wasted bandwidth from forcing partially-used
segment files to be archived. Note that the standby servers'
<varname>restore_command</> scripts still deal in whole WAL files,
so the incrementally copied data is not ordinarily made available to
the standby servers. It is of use only when the primary dies —
then the last partial WAL file is fed to the standby before allowing
it to come up. So correct implementation of this process requires
cooperation of the <varname>restore_command</> script with the data
copying program.
</para>
</sect2>
<sect2 id="backup-incremental-updated">
<title>Incrementally Updated Backups</title>
<indexterm zone="backup">
<primary>incrementally updated backups</primary>
</indexterm>
<indexterm zone="backup">
<primary>change accumulation</primary>
</indexterm>
<para>
In a warm standby configuration, it is possible to offload the expense of
taking periodic base backups from the primary server; instead base backups
can be made by backing
up a standby server's files. This concept is generally known as
incrementally updated backups, log change accumulation or more simply,
change accumulation.
</para>
<para>
If we take a backup of the standby server's files while it is following
logs shipped from the primary, we will be able to reload that data and
restart the standby's recovery process from the last restart point.
We no longer need to keep WAL files from before the restart point.
If we need to recover, it will be faster to recover from the incrementally
updated backup than from the original base backup.
</para>
<para>
Since the standby server is not <quote>live</>, it is not possible to
use <function>pg_start_backup()</> and <function>pg_stop_backup()</>
to manage the backup process; it will be up to you to determine how
far back you need to keep WAL segment files to have a recoverable
backup. You can do this by running <application>pg_controldata</>
on the standby server to inspect the control file and determine the
current checkpoint WAL location.
</para>
</sect2>
</sect1>
<sect1 id="migration">
<title>Migration Between Releases</title>
<indexterm zone="migration">
<primary>upgrading</primary>
</indexterm>
<indexterm zone="migration">
<primary>version</primary>
<secondary>compatibility</secondary>
</indexterm>
<para>
This section discusses how to migrate your database data from one
<productname>PostgreSQL</> release to a newer one.
The software installation procedure <foreignphrase>per se</> is not the
subject of this section; those details are in <xref linkend="installation">.
</para>
<para>
As a general rule, the internal data storage format is subject to
change between major releases of <productname>PostgreSQL</> (where
the number after the first dot changes). This does not apply to
different minor releases under the same major release (where the
number after the second dot changes); these always have compatible
storage formats. For example, releases 7.2.1, 7.3.2, and 7.4 are
not compatible, whereas 7.2.1 and 7.2.2 are. When you update
between compatible versions, you can simply replace the executables
and reuse the data directory on disk. Otherwise you need to back
up your data and restore it on the new server. This has to be done
using <application>pg_dump</>; file system level backup methods
obviously won't work. There are checks in place that prevent you
from using a data directory with an incompatible version of
<productname>PostgreSQL</productname>, so no great harm can be done by
trying to start the wrong server version on a data directory.
</para>
<para>
It is recommended that you use the <application>pg_dump</> and
<application>pg_dumpall</> programs from the newer version of
<productname>PostgreSQL</>, to take advantage of any enhancements
that might have been made in these programs. Current releases of the
dump programs can read data from any server version back to 7.0.
</para>
<para>
The least downtime can be achieved by installing the new server in
a different directory and running both the old and the new servers
in parallel, on different ports. Then you can use something like
<programlisting>
pg_dumpall -p 5432 | psql -d postgres -p 6543
</programlisting>
to transfer your data. Or use an intermediate file if you want.
Then you can shut down the old server and start the new server at
the port the old one was running at. You should make sure that the
old database is not updated after you run <application>pg_dumpall</>,
otherwise you will obviously lose that data. See <xref
linkend="client-authentication"> for information on how to prohibit
access.
</para>
<para>
In practice you probably want to test your client
applications on the new setup before switching over completely.
This is another reason for setting up concurrent installations
of old and new versions.
</para>
<para>
If you cannot or do not want to run two servers in parallel you can
do the backup step before installing the new version, bring down
the server, move the old version out of the way, install the new
version, start the new server, restore the data. For example:
<programlisting>
pg_dumpall > backup
pg_ctl stop
mv /usr/local/pgsql /usr/local/pgsql.old
cd ~/postgresql-&version;
gmake install
initdb -D /usr/local/pgsql/data
postgres -D /usr/local/pgsql/data
psql -f backup postgres
</programlisting>
See <xref linkend="runtime"> about ways to start and stop the
server and other details. The installation instructions will advise
you of strategic places to perform these steps.
</para>
<note>
<para>
When you <quote>move the old installation out of the way</quote>
it might no longer be perfectly usable. Some of the executable programs
contain absolute paths to various installed programs and data files.
This is usually not a big problem but if you plan on using two
installations in parallel for a while you should assign them
different installation directories at build time. (This problem
is rectified in <productname>PostgreSQL</> 8.0 and later, but you
need to be wary of moving older installations.)
</para>
</note>
</sect1>
</chapter>
|